Oligoryzomys flavescens

Karyotype: 2n = 64 and FN = 66. Autosomal complement: two small metacentric and submetacentric pairs, and 29 acrocentric pairs (one very large and the remaining large to small decreasing in size). Sex chromosomes: X, a large submetacentric that presented a variable centromeric index; Y chromosome presented three different morphologies, a medium metacentric, a medium submetacentric, and a small acrocentric (Yonenaga et al. 1976; Myer & Carleton 1981, pp. 16, Fig. 5C; Brum-Zorrilla et al. 1988; Rioja et al. 1988; Espinosa & Reig 1991; Sbalqueiro et al. 1991; Aniskin & Volobouev 1999; Andrades-Miranda et al. 2001a; Weksler & Bonvicino 2005; Di-Nizo et al. 2015). A different diploid number from 65 to 68 were reported due to the presence of 1 to 4 small acrocentric or metacentric supernumerary chromosomes, mosaicism karyotypes with 2n = 64/64+1B and 2n = 64+1B/ 64+2Bs were also reported. These variations in number and presence of supernumerary chromosomes were widely distributed throughout the karyotyped specimens (Table 9, Fig. 19; see Yonenaga et al. 1976; Myer & Carleton 1981; Brum-Zorrilla et al. 1988; Rioja et al. 1988; Espinosa & Reig 1991; Sbalqueiro et al. 1991; Aniskin & Volobouev 1999; Andrades-Miranda et al. 2001a; Weksler & Bonvicino 2005; Di-Nizo et al. 2015). Additional chromosome polymorphisms, reported only for samples from Minas Gerais, state of Brazil, were due to a whole heterochromatic arm occurring in 1 to 3 large to medium-sized acrocentric chromosomes, and a fundamental number of 68 possibly due to a pericentric inversion affecting one small autosomal pair (Yonenaga et al. 1976; Brum-Zorrilla et al. 1988; Rioja et al. 1988; Espinosa & Reig 1991; Bonvicino & Weksler 1998; Aniskin & Volobouev 1999; Sbalqueiro et al. 1991; Weksler & Bonvicino 2005). C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes. The X chromosome has a heterochromatic short arm, which was responsible for the observed variable centromeric indexes. The metacentric or submetacentric Y chromosomes presented over half their short arm heterochromatic, while the acrocentric Y chromosome was entirely heterochromatic. The B chromosomes were entirely C-positive, but the staining was less intense in some cases (Rioja et al. 1988; Espinosa & Reig 1991; Sbalqueiro et al. 1991; Aniskin & Volobouev 1999). G-banding was also performed (Brum-Zorrilla et al. 1988; Espinosa & Reig 1991; Sbalqueiro et al. 1991; Aniskin & Volobouev 1999; Di-Nizo et al. 2015). Multiple NORs, varying from two to eight were localized at the telomeric regions of the short arms of small acrocentric pairs (Sbalqueiro et al. 1991).